The present invention relates to an electromagnetic detection system comprising transmission means producing an electromagnetic interrogation field in at least one first detection zone; a plurality of responders having attachment means for attachment to articles to be protected, each including a resonant circuit and, if present in a detection zone, in response to the interrogation field, producing a signal; and first detection means adapted to detect a responder and, on detection of a responder, to produce a warning signal.
Similar detection systems are already known in many embodiments and are mostly used to prevent shop-lifting. The goods to be protected are then each provided with a responder, which is difficult to remove without special tools. When a protected article is purchased, the responder is removed at the cash-desk or deactivated in another manner. At the exit of the shop, the transmission means create interrogation zones which have to be passed for the customer to be able to leave the shop. If an article still carrying a responder is passed through an interrogation zone, this will be detected by the detection means and a warning signal is produced.
In the simplest form of such a system, the transmission means produce an interrogation field having a single frequency identical to the resonate frequency of a single resonant circuit present in each responder. When a responder is present in the interrogation field, the resonant circuit will arrive in the resonant state. The resonant circuit then forms an additional load for the transmission means, which can be detected at the side of the transmission means. In that case we speak of an absorption system. A resonant circuit being in the resonant state also transmits itself a signal, which can be detected by special receiving means. In that case we speak of a transmission system. In a transmission system the responder may alternatively be arranged in such a manner that, in response to an interrogation field, it transmits a signal at one or more predetermined frequencies which differ from the frequency of the interrogation field. This can be effected by means of a frequency divider or multiplier in the responder or by incorporating in the resonant circuit of a responder a non-linear element, such as a diode.
Furthermore, use can be made both in a transmission system and in an absorption system of an interrogation field having a periodically swept frequency. The resonant frequency of the responders then lies within the frequency sweep range of the interrogation field.
A drawback of the prior art detection systems is that these only respond if articles unpaid-for and still provided with an operative responder are taken through an interrogation field. It frequently occurs that it is tried to remove a responder from a protected article already inside the shop, e.g. in the toilets or in the fitting room, by means of tools in an unauthorized manner with the intent to take away this article imperceptibly.
To prevent this, the attachment means, mostly consisting of a hardened steel pin with a broad head inserted through the material of the article to be protected in a ball lock, must be very robust and hence expensive and heavy, so that removal without the use of the special tool is impeded.
It is also tried sometimes to damage the resonant circuit, mostly embedded in a synthetic plastics housing. Such a damaged responder cannot be detected by the prior art systems either. In the past, it had been tried to solve this problem by making the responders more robust. This too, renders the responders more expensive and heavier. The latter again is a drawback when responders are used on vulnerable articles, e.g. clothing of fine materials.